Load drivers are known in which a semiconductor switch receives control signals at a control electrode and has an output electrode connected to a load for control of the load. When the load is inductive, such as would be the case if the load is a relay in an anti-skid brake system, the load will provide a flyback or inductive kick voltage when load current is interrupted.
Some prior load driver systems have used a controller to determine, via a command line, the control signals provided to the control electrode of the semiconductor switch. Then, on the same command line, the controller monitors logic states which indicate the operation of the semiconductor switch and its load. This minimizes the number of lines connecting the controller and load driver. Typically, the output of the semiconductor switch is monitored when the switch is on and also when the switch is off. This provides a sequence of two logic states during the on and off states to be analyzed by the controller for diagnosing various types of faults. However, in some load driver systems it is imperative that a more complex and thorough diagnostic analysis of the operation of the semiconductor switch and load be implemented. For example, when the load comprises a relay device in an anti-skid brake system, it is important to know exactly what type of fault mode has occurred. For some types of fault modes a fail-safe or limp home mode can be implemented which still allows operation of the vehicle in which the anti-skid brake system is provided. However, for other types of faults of the anti-skid brake relay, the entire brake system must be deactivated because it is unreliable. Prior systems which use a common command/fault line have not been able to identify a large number of different faults.
Some systems utilize multiplexing to minimize the number of lines connecting a microprocessor controller and a plurality of load drivers. U.S. Pat. No. 4,932,246 to Deutsch et al. entitled, "Diagnostic Fault Test System and Circuit" illustrates such a system which also includes diagnostic fault testing apparatus. That system identifies a large number of faults, but it uses a fault sense line separate from the command control line. Systems which use a common command/fault line fail to adequately discriminate between various different types of fault conditions.